Technical Field
The present disclosure relates to photodiodes and particularly to monolithic photodetectors sensitive to radiation in at least two different wavelength bands ranging from ultraviolet to infrared.
Description of the Related Art
Photodiodes are semiconductor devices that contain a P-N junction, and often an intrinsic (undoped) layer between n and p layers. Devices with an intrinsic layer are called P-i-N photodiodes. Alternatively, a Schottky photodiode uses a metal-semiconductor junction as a Schottky junction rather than a semiconductor-semiconductor junction as in conventional photodiodes. Light absorbed in depletion regions or the intrinsic region in P-i-N photodiodes generates electron-hole pairs, most of which contribute to a photocurrent. Conventional photodiodes operate in different regions of the electromagnetic radiation spectrum. The particular semiconductor materials that make up the photodiode determine the particular wavelength or wavelength range of the radiation to which the photodiode responds. Photodiodes can be fabricated from elemental semiconductors, such as silicon, as well as compound semiconductors, such as gallium-arsenide, gallium nitride, or silicon carbide.
Broadband photodetectors having detection capabilities ranging from ultraviolet to near infrared can be useful in many applications such as data transport and storage in optical communications. Moreover there are several applications, mainly in military and industrial domains, which aspire to simultaneous or spatially synchronized detection of optical photons in different spectral regions. Photons emitted by fires, jet or rocket nozzles or stellar luminaries for example have typical wavelengths ranging from ultraviolet to infrared. All these emissions are detected over the ambient light background by two or more detectors having different sensitivity ranges, or better by a single fast multi-range photodetector giving the possibility to perform time resolved measurements in different optical bands. These known devices can have false-alarm warnings. To avoid the false-alarm warnings it is important to know an appropriate photodetector spectral range and appropriate device speed and spatial resolution. For all these applications, photomultiplier tubes (PMT) are typically used as photodetectors. For instance Hamamatsu PMT UV Tron R2868 is commonly used for flame and fire alarm detection. However, although PMTs have high sensitivity and good timing response, these devices are bulky, expensive, sensitive to magnetic fields, require high operating bias, and have low mechanical and temperature strength.
Another possibility could be the use of two or more discrete solid state detectors having different sensitivity ranges in a single housing. However, this solution does not allow for detection of multiband optical signals with high spatial resolution and, furthermore, metallic or ceramic packages used to house these devices cannot withstand temperatures as high as those achieved in flame monitoring systems. An additional possibility could be the use of different external (and expensive) interferential filters to make a single broadband photodiode sensitive only in selected portions of the electromagnetic spectrum. However, this solution typically implies the use of a broadband photodetector and, what's more, does not allow a fast detection of photons with different wavelengths due to the use of the external filters usually housed in appropriate filter wheels controlled, for instance, by remote.
Moreover the capability of using a single detector as a receiver makes the detection system less expensive and more compact, and, not least, could make it easier for the system vendors to reduce the inventory.
A low operating bias is particularly relevant in low power consumption applications. Electro-tunable multiband photodiodes have known a growing success in last years. The optical absorption of these devices realized by stacking multilayers of different materials with different optical properties and energy gaps (i.e., high cut off wavelength) are modulated by varying the applied reverse bias.
The possibility to modulate the absorption and eventually increase the photodetector sensitivity wavelength range by changing the applied reverse bias has been found to be a powerful tool to reduce the overall number of photodetectors used for the detection of photons in a wide wavelength range.
A multiband spectral infrared photodetector and imager are disclosed by Mitra in US Pat. App. No. 2004/0108461, in which two or more different bands in the infrared wavelength range are detected by a diffractive resonant optical cavity. Furthermore, a device and the relative fabrication method for a two-color infrared detector are disclosed by Park et al. in U.S. Pat. No. 6,049,116, and a multilayer junction photodiode for multiple infrared wavelengths detection is presented by Dutta in US Pat. App. No. 2009/0189207.
On the other hand, Tsang, in US Pat. App. No. 2009/0159785 presents an optical sensing device with multiple photodiode elements and multi-cavity Fabry-Perot ambient light structure in order to detect light signals with different wavelength spectrums.
Korona et al., in the paper “Multiband GaN/AlGaN UV Photodetector, Acta Physica Polonica A, Vol. 110, No 2, pp. 211-217, 2006,” present a multiband GaN/AlGaN photodetector structure capable of detecting three UV ranges, tuned by external voltage. The multilayer structures were grown by a Metal Organic Chemical Vapor Deposition (MOCVD) technique, while the device was designed as a Schottky photodiode with a semitransparent continuous gold Schottky contact evaporated on its surface.
However, at the moment these devices experience a poor technological maturity, as is highlighted by their high leakage (dark) current and often by a not optimized control of the optical absorption (i.e., the photoresponse) in different wavelength bands with the bias.